Bond-over-active circuity gallium nitride devices

ABSTRACT

Implementations of semiconductor devices may include: a first layer with a plurality of cells, each cell having a drain finger, a source finger and a gate ring; a second layer having a drain pad and a source pad, the drain pad having a width and a source pad having a width substantially the same as the drain pad; wherein a width of each drain finger of the first layer is wider than a width of each source finger of the first layer; and wherein each drain pad is coupled to each drain finger through a first contact and the source pad is coupled to each source finger through a second contact, where a width of the first contact is wider than a width of the second contact.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of the earlier U.S. UtilityPatent Application to Woochul Jeon entitled “Bond-Over-Active CircuitryGallium Nitride Devices,” application Ser. No. 15/214,579, filed Jul.20, 2016, now pending, the disclosure of which is hereby incorporatedentirely herein by reference.

BACKGROUND 1. Technical Field

Aspects of this document relate generally to semiconductor devices.Specific implementations involve cascode devices.

2. Background

Gallium nitride (GaN) devices are used for high power or high frequencysemiconductor devices. GaN devices include source and drain fingers andare operated using a gate.

SUMMARY

Implementations of semiconductor devices may include: a first layer witha plurality of cells, each cell having a drain finger, a source fingerand a gate ring; a second layer having a drain pad and a source pad, thedrain pad having a width and a source pad having a width substantiallythe same as the drain pad; wherein a width of each drain finger of thefirst layer is wider than a width of each source finger of the firstlayer; and wherein each drain pad is coupled to each drain fingerthrough a first contact and the source pad is coupled to each sourcefinger through a second contact, where a width of the first contact iswider than a width of the second contact.

Implementations of semiconductor devices may include one, all, or any ofthe following:

The plurality of cells may be gallium nitride (GaN) high electronmobility transistors (HEMT).

Implementations of semiconductor devices may include: a first cellhaving a first plurality of fingers and a second cell having a secondplurality of fingers; a first drain pad coupled to the first pluralityof fingers; a second drain pad coupled to the second plurality offingers; a source pad coupled to the first plurality of fingers and thesecond plurality of fingers; a gate bus located between the firstplurality of fingers and the second plurality of fingers; and at leastone gate pad coupled to the gate bus.

Implementations of semiconductor devices may include one, all, or any ofthe following:

The semiconductor device may further include a second gate pad coupledto the gate pad on an end of the gate bus opposing an end to which theat least one gate pad is coupled.

The source pad may be coupled to a substrate of a second transistorforming a cascode device.

The second transistor may be a silicon metal-oxide field effecttransistor (Si-MOSFET) having a Si substrate.

The Si substrate of the Si-MOSFET may be coupled to a conductive filmand the conductive film may be coupled to a drain of the Si-MOSFET.

Gate vias may be used to couple the source pad to the second transistor.

The gate bus and the gate pad may be covered in a passivationprocess/layer and the source pads and the drain pads may be open.

Implementations of semiconductor devices may include: a plurality ofcells, each cell having at least one drain finger, at least one sourcefinger and at least one gate; at least one drain pad; a plurality ofsource pads; and a gate bus coupled with the plurality of cells; and atleast one gate pad coupled to the gate bus; wherein the drain pad iscoupled to the at least one drain finger of each of the plurality ofcells; and wherein each of the plurality of source pads is coupled to atleast one source finger of each of the plurality of cells.

Implementations of semiconductor devices may include one, all, or any ofthe following:

The plurality of cells may be gallium nitride (GaN) high-electronmobility transistors (HEMT).

The plurality of cells may include a first group having a first lengthand a second group having a second length that is longer than the firstlength of the first group.

The at least one drain pad may be positioned over the second group ofcells having the second length.

A second drain pad may be positioned opposite the at least one drain padwith the plurality of source pads between, the second drain pad coupledto the at least one drain pad through at least one drain bus.

The gate bus may be located under the plurality of source pads.

The plurality of source pads may be electrically coupled together duringpackaging.

The plurality of source pads may be coupled to one another through wirebonding.

Implementations of semiconductor devices may include: a first metallayer having an active area and a non-active area; and a second metallayer having at least one drain pad, at least one source pad and atleast one gate pad; wherein the active area includes a plurality ofsource fingers and a plurality of drain fingers interdigitated with oneanother; and at least one gate finger; wherein the drain pad is coupledto the plurality of drain fingers at one end of the plurality of drainfingers; wherein the drain pad is located only in the non-active area ofthe first metal layer; and wherein the first metal layer is electricallycoupled to the second metal layer.

Implementations of semiconductor devices may include one, all, or any ofthe following:

The plurality of drain fingers may be coupled together at one end.

The semiconductor device may further include a single contact betweenthe drain pad and the plurality of drain fingers.

The source pad may be located only in the active area of the first metallayer.

The semiconductor device may further include a second source pad andwherein the drain pad may be located between the source pad and thesecond source pad.

The semiconductor device may further include a second drain pad andwherein the source pad may be located between the drain pad and thesecond drain pad.

The foregoing and other aspects, features, and advantages will beapparent to those artisans of ordinary skill in the art from theDESCRIPTION and DRAWINGS, and from the CLAIMS.

BRIEF DESCRIPTION OF THE DRAWINGS

Implementations will hereinafter be described in conjunction with theappended drawings, where like designations denote like elements, and:

FIG. 1A-1C show the relationship between finger length and voltage in abond-over-active (BOA) device implementation;

FIG. 2 is a top view of a BOA device;

FIG. 3A is a schematic of an implementation of a semiconductor devicehaving a drain-source-drain (DSD) pad;

FIGS. 3B and 3C are top views of implementations of semiconductordevices having a DSD pad;

FIG. 4A is a top view of another implementation of a semiconductordevice having a BOA structure;

FIG. 4B is a top view of the pad layer from the semiconductor device inFIG. 4A;

FIG. 4C is a top view of the high electron mobility transistor (HEMT)cell from the semiconductor device in FIG. 4A;

FIG. 5A is a cross sectional view of a transistor;

FIG. 5B is a cross sectional view of an implementation of a transistorwith an enlarged drain contact;

FIG. 6A is a top view of an implementation of a semiconductor devicehaving a DSD BOA structure with a gate via to a silicon substrate;

FIG. 6B is a perspective view of an implementation of a cascode using animplementation of the device illustrated in FIG. 6A;

FIG. 7A is a top view of an implementation of semiconductor having amulti-source BOA pad;

FIG. 7B is a top view of the pad from FIG. 7A;

FIG. 8A-8C are examples of different implementations semiconductordevices having multi-source BOA pads;

FIG. 9A-9C are enlarged views of a semiconductor device having amulti-source BOA pad;

FIG. 10A is an implementation of a semiconductor device having a BOAstructure;

FIG. 10B-10C are schematics showing the concentration of electric fieldsin an implementation of a semiconductor device having a BOA structure;

FIG. 11A is a top view of an implementation of a semiconductor devicehaving a semi-BOA structure;

FIG. 11B is a schematic of a top view of the device from FIG. 11A;

FIG. 11C is a top perspective view of an implementation of asemiconductor device having a semi-BOA structure;

FIG. 12A is a top view of an implementations of a semiconductor devicehaving a semi-BOA structure;

FIG. 12B is a top view of another implementation of a semiconductordevice having a semi-BOA structure;

FIG. 13A is an implementation of a semiconductor device having asemi-BOA structure;

FIG. 13B is an enlarged view of a section of the device from FIG. 13Alabeled 13B;

FIG. 13C is an enlarged view of a section of the device from FIG. 13Blabeled 13C;

FIG. 13D is an enlarged view of a section of the device from FIG. 13Blabeled 13D; and

FIG. 13E is a top view of the pad metal from the device in FIG. 13A.

DESCRIPTION

This disclosure, its aspects and implementations, are not limited to thespecific components, assembly procedures or method elements disclosedherein. Many additional components, assembly procedures and/or methodelements known in the art consistent with the intended semiconductordevices will become apparent for use with particular implementationsfrom this disclosure. Accordingly, for example, although particularimplementations are disclosed, such implementations and implementingcomponents may comprise any shape, size, style, type, model, version,measurement, concentration, material, quantity, method element, step,and/or the like as is known in the art for such semiconductor devicesand implementing components and methods, consistent with the intendedoperation and methods.

Referring to FIG. 1A, an implementation of a device bond over active(BOA) gallium nitride (GaN) high electron mobility transistor (HEMT)device 2 is illustrated. A typical BOA device has a first layer withalternating drain fingers 4 and source fingers 6 and a second layer witha drain pad 8 coupled to the drain fingers 4 and a source pad 10 coupledto the source fingers 6. The length of a finger (Lf) 12 is measured fromthe top of one pad to the top of the next pad. For example, the lengthof finger 12 is measured from the top of the source pad 10 to the top ofthe drain pad 10.

The finger length of the drain and source is an important parameter thatinfluences chip size and on resistance (R_(on)) of the device,especially for BOA devices. Shorter finger length and lower finger metalresistance may improve current capacity of a device. Because ofdifficulties in fabrication, the thin metal process, some finger lengthsare not long enough to achieve a R_(on) of less than 100 milliOhms(mOhm). To achieve a longer finger length, thicker metal processes canbe used but there is a limitation to the thickness that can be usedbecause the inter-metal dielectric (IMD) process involved can be verycomplicated.

Referring now to FIG. 1B, a graph of the voltage drop across the fingersof a device is illustrated with a finger thickness of 1.5 microns. Thegraph illustrates the change in voltage drops as the width of the fingeris increased from 4 microns to 12 microns. Voltage drop can be improvedas the Ron is improved. If a 1.5 micron thick and a 4 micron wide metalfinger is used, the finger length is limited to approximately 300microns if nor more than a 10% voltage drop at the end of the finger isdesired. If the finger width is 10 microns, the finger length can beapproximately 500 microns. Increasing the finger thickness may help todecrease the voltage drop for the same finger length. Referring to FIG.1C, for example, the voltage drop across fingers with a thickness of 4microns is illustrated. This graph illustrates a smaller voltage dropacross the same length and width of fingers as illustrated in FIG. 1B.As described above, there is a limit to increasing the thickness of thefingers to achieve an equivalently longer finger length because the IMDprocess involved can be very complicated.

Referring to FIG. 2, a device having a BOA structure using a doublemetal process is illustrated. On the first metal layer there are sourcefingers 16 and drain fingers 18. On the second metal layer there aresource pads 20 and drain pads 22. For example, to achieve low resistancebetween the drain and source when the device is on (Rdson), the fingerlength must be shorter than 1 mm for a finger having a width of 6 μm andthickness of 4 μm. A longer finger length may be achieved by increasingthe size of the source pad. For example, referring now to FIG. 3A, a 60mOhm device 26 may be created by connecting two 120 mOhm devices 24.This leads to a drain-source-drain (DSD) pad structure having a largesource pad and two drain pads on either side as illustrated in FIGS. 3Band 3C. By non-limiting example, FIGS. 3B and 3C illustrate at least twoconfigurations of gates that may be utilized with a DSD pad structure.

Referring now to FIGS. 4A and 4C, an implementation of a semiconductordevice having a DSD pad structure 28 is illustrated. On the devicelayer, two GaN HEMT cells 30 with short fingers are positioned back toback in series. The large source pad 32 of the DSD pad connects thesource fingers 34 of the two GaN cells 30 through contacts. The sourcepad 32 is coupled to the source fingers 34 through contacts 31 betweenthe ohmic metal and pad metal. The drain pads 36 located on either sideof the source pad 32 are coupled to the drain fingers 38 of the HEMTcells 30 through contacts 39. A gate bus 40 is located between the HEMTcells 30 and is coupled to the gate pads 40 located on either side ofthe source pad 32. Referring now to FIG. 4B, a view of the DSD padstructure of FIG. 4A is illustrated. The drain pads 36 are located aboveand below the source pad 32. The gate pads 42 are located to the rightand left of the source pad 32. Referring to FIG. 4C, a single GaN HEMTcell is illustrated. The drain fingers 38 are located near the edges ofthe cell. The source finger 34 is located in the middle of the cell. Thegate fingers 44 are located to the right and left of the source finger34.

Referring now to FIG. 5A-5B, a cross-sectional view of the contacts of atransistor cell implementation is illustrated. In FIG. 5A, a device isillustrated. Generally, increasing the size of the contacts is one wayto lower the resistance of the fingers. The source ohmic contact 48 tothe channel of the source finger 46 is wider than that of the drainohmic contact 52 of the drain finger 50. This is because the sourcefinger can extend over the gate fingers 54 as well as over the sourcecontact. The drain finger 50 is limited by the size of the drain contact52. By extending the drain ohmic contact length 38, the width of thedrain finger ohmic contact 52 can be increased. With a wider draincontact length 52, the width of the drain finger 50 can be increased tobe substantially the same or the same as the width of the source finger46. Widths that are substantially the same are similar or essentiallythe same in size. By non-limiting example, one way to increase thelength of the drain finger is to connect two HEMT cells in series asillustrated in FIG. 4A. Increasing the width of drain finger may requirea wider drain contact 50 because of the high potential at the edge ofthe extended drain metal. Referring now to FIG. 5B, the drain contact isincreased to increase the finger 50 of the drain region (see the dottedarrow in the figure) and the drain contact 52 can be widened. Thisprocess limits current crowding.

Referring now to FIGS. 6A and 6B, an implementation of a semiconductordevice having a DSD BOA pad structure with a gate via to a siliconsubstrate is illustrated. The DSD BOA pad 58 can couple to a siliconmetal-oxide semiconductor field effect transistor (MOSFET) device by athrough via. The gate 62 of the DSD device 58 is connected to thesubstrate/source 64 of a Si-MOSFET to create a cascode device 56. Thiscombination forming a cascode device provides a gate for the Si-MOSFETdevice.

Referring now to FIG. 7A, an implementation of a semiconductor device 66having a multi-source BOA pad is illustrated. This BOA structure may beimplemented when using longer fingers in the cells of transistors.Shorter fingers may be useful in lowering the on resistance of a devicebut longer fingers help to make the aspect ratio of the device better.For example, if the maximum finger length used is 1 mm due to thelimited metal thickness, the aspect ratio of 100 mOhm device can behigher than 3:1 (and a 50 mOhm device has 6:1 aspect ratio), whichcannot be used for actual manufacturing. In this view, the first metallayer (M1) 68 can be seen having a plurality of cells 70 each having adrain finger, a source finger and a gate. The second metal layer (M2) 72and 75 of the device 66 has a first drain pad having as opening 74 andmultiple isolated source pads having openings 76. The first drain padopening 74 couples to the drain fingers 70 of the M1 layer 68 and thesource pad openings 76 couple to each of the source fingers 70 of the M1layer 68. In various implementations, the plurality of cells 70 mayinclude a first group and a second group, the second group having alonger length than the first group. The first drain pad opening 74 maycouple to the second group of drain fingers having the longer length.There may be, in various implementations, a second drain pad on the M2layer. This second drain pad does not couple directly to the M1 layer,rather the second drain pad is connected to the first drain pad throughdrain fingers 78 on the M2 layer located between the source pads 76.These M2 drain fingers 78 may be non-rectangular in shape for bettercurrent spreading. The multiple isolated source pads may be coupledduring the packaging process by wire bonding, clip bonding, or anysuitable bonding known in the art. Referring now to FIG. 7B, a top downview of the M2 layer 72 of the device is illustrated. The first drainpad 75, drain fingers 78 and second drain pad 80 are coupled together.The source pads 77 are located between the drain finger 78.

Referring now to FIGS. 8A-8C, various implementations of semiconductordevices with multi-source pads are illustrated. Referring to FIG. 8A, asemiconductor device 82 with nine source pads 84 is illustrated. A firstgroup 86 of a plurality of cells/fingers having a first length and asecond group 88 of a plurality of cells/fingers having a second lengthis illustrated. The second length 88 is longer than the first length 86.At least one drain pad 91 is positioned over the second, longer groupsof cells 88. A second drain pad 92 is positioned opposite the at leastone drain pad 91 with the plurality of source pads 84 between the drainpads 92 and 91. The second drain pad 92 is coupled to the at least onedrain pad 91 through at least one drain bus 94. The drain pads 91 and 92are coupled to the cells/fingers 88 and 86 through the drain pad opening90. A gate bus (hidden under the pad structures) is located under theplurality of source pads 84 and is coupled to a gate pad 96 located nearthe source pads.

Referring to FIG. 8B, an implementation of a multi-source pad metallayout 98 is illustrated. In this particular layout implementation, theM1 layer is shown having seven source pads 100 open for bonding and onegate pad 102. Referring to FIG. 8C, an implementation of a semiconductorpackage 106 is illustrated. The multiple source pads 108 may beelectrically coupled together by a wire 110, a clip and any similarmethod known in the art. The drain pad 112 and the gate pad 114 may eachbe packaged with, by non-limiting example, wires 116 and 118, clips andsimilar methods.

Referring now to FIGS. 9A-9C, an enlarged view of the device from FIG.8A is illustrated. In FIG. 9A, a portion of the at least one drain pad120, two full and two partial source pads 122, a portion of the seconddrain pad 124, and the drain buses 126 coupling the second drain pad 124to the drain pad 120 is illustrated. In FIG. 9B, an enlarged view of aportion of FIG. 9A showing the plurality of fingers of the semiconductordevice 82 is illustrated. In this view, a drain finger 128 of the deviceis visible. A gate 130 is located near the source finger 132. Referringto FIG. 9C, another enlarged view is illustrated. In this view, thedrain feed 126 coupling the at least one drain pad 120 and the seconddrain pad 124 is illustrated. The contacts 136 coupling the open drainpad 120 and source pads 122 to the plurality of fingers 88 and 86 arealso visible. The gate bus 138 is illustrated under the source pads 122.

Referring now to FIG. 10A-10C, a semiconductor device 140 having a BOApad structure is illustrated. This device has interdigitated sourcefingers 142 and drain fingers 144. A drain pad 146 is coupled to thedrain fingers 144 through contacts 148. A source pad 150 is coupled tothe source fingers 142 through contacts 152. In this particularimplementation, a gate pad 154 is positioned between the drain pad 146and source pad 150. In various implementations of a BOA design, the gatepad may be located anywhere near the source pad in other orientationsother than the one illustrated in the figures. As illustrated in FIG.10B, there is a high electric field concentration on the edge of thesource fingers 142 and the gate 156 due to the high electrical potentialof the drain region of the device 140. As illustrated in FIG. 10C, thesource pad 150 may act as a field plate to relieve the electric field atthe edge of the source finger 142 or gate 156.

Referring now to FIGS. 11A-11C, an implementation of a semiconductordevice 158 having a semi-BOA design is illustrated. The device 158 has afirst layer having interdigitated drain fingers 160 and source fingers162. A drain pad 164 is located over a non-active area of the device 158and is coupled to the drain fingers 160 through a single contact 166.However, in various implementations, single or multiple vias may also beused. A source pad 168 is located in the active area of the device 158.The source pad 168 is coupled to the source fingers 162 through contacts170 electrically coupling each source finger 162 to the source pad 168.A gate pad 172 is located between the drain pad 164 and the source pad168. The gate pad is coupled to a gate bus 174. Referring to FIG. 11C, athree dimensional perspective view of an implementation of the device158 is illustrated. In this view, the intermetal dielectric (IMD)between the fingers 160 and 162 and pads 164, 168 and 172 is visible andpart of the source pad 168 has been removed to show structure.

A semi-BOA design may involve a less difficult manufacturing processbecause the IMD layer can be thinner when the source pad 168 acts as afield plate. This design may also provide a better pad layout forpackaging and better current spreading than a non-BOA device because thefingers can be thinner and shorter. With this design it may be possibleto stack a silicon field effect transistor (Si-FET) die/device on thesource pad 168. This design may allow for a less complicated and morereliable IMD process than a full-BOA device 140. Table 1 below shows theadvantages of a semi-BOA structure comparted to both a non-BOA and fullBOA structure. Though the active area is smaller in a semi-BOA structurethan a full-BOA structure because the dielectric is thinner, the IMDprocess is less complicated and the device is more reliable.

TABLE 1 Non BOA Semi BOA BOA Active area ~60% ~70% ~85% IMD  <1 um 2~3um  >3 um Finger size ~10 um  ~5 um 2~5 um Si FET stack NA OK OK Clippkg . . . OK OK

Referring now to FIGS. 12A and 12B, possible layouts of semi-BOAsemiconductor device implementations are illustrated. In FIG. 12A, thesource pad 178 is located between the drain pad 180 and a second drainpad 182. The gate pad 184 is located adjacent to the source pad 178.Referring to FIG. 12B, the drain pad 180 in this implementation islocated between the source pad 178 and a second source pad 186. In thisimplementation there are two gate pads located on either side of thedrain pad 180 and between the two source pads 178 and 186. In bothimplementations, FIG. 12A and FIG. 12B, the drain pads are located overthe non-active area of the first layer.

Referring now to FIGS. 13A-13E, an implementation of a semiconductordevice 188 having a semi-BOA layout is illustrated. The drain pad 190 islocated over the non-active area of the device while the source pad 192and gate pads 194 are located over the active area of the device 188(see also FIG. 13E). In FIG. 13B, an enlargement of an area from FIG.13A is illustrated. The bottom of the drain pad 190 near the drainfingers 196 is shown. An enlargement of this area is shown in FIG. 13C.The drain finger 196 extends from the drain pad 190 to the under thesource pad 192, as illustrated in FIG. 13D. In FIG. 13C, the drainfinger is formed from ohmic metal, the first metal layer, and the secondmetal layer. The portion of the drain finger under the source pad may beformed from ohmic metal and the first metal layer. The top of the sourcepad 192 is also shown.

Various methods of manufacture for the semiconductor deviceimplementations can be devised by those of ordinary skill in the artusing the principles disclosed in this document.

In places where the description above refers to particularimplementations of semiconductor devices having bond over active padstructures and implementing components, sub-components, methods andsub-methods, it should be readily apparent that a number ofmodifications may be made without departing from the spirit thereof andthat these implementations, implementing components, sub-components,methods and sub-methods may be applied to other semiconductor devices.

What is claimed is:
 1. A semiconductor device comprising: a first layerwith a plurality of cells, each cell comprising a drain finger, a sourcefinger and a gate ring; and a second layer comprising a drain pad and asource pad, the drain pad having a width and a source pad having a widthsubstantially the same as the drain pad; wherein a width of each drainfinger of the first layer is wider than a width of each source finger ofthe first layer; wherein each drain pad is coupled to each drain fingerthrough a first contact and the source pad is coupled to each sourcefinger through a second contact, where a width of the first contact iswider than a width of the second contact; and wherein one of the drainpad or the source pad is positioned over one of the drain finger or thesource finger.
 2. The semiconductor device of claim 1, wherein theplurality of cells are gallium nitride (GaN) high electron mobilitytransistors (HEMT).
 3. A semiconductor device comprising: a first cellcomprising a first plurality of fingers and a second cell comprising asecond plurality of fingers; a first drain pad coupled to the firstplurality of fingers; a second drain pad coupled to the second pluralityof fingers; a source pad coupled to the first plurality of fingers andthe second plurality of fingers; a gate bus located between the firstplurality of fingers and the second plurality of fingers; and at leastone gate pad coupled to the gate bus; wherein one of the first drainpad, the second drain pad, the source pad, or the gate pad is positionedover one of the first plurality of fingers, the second plurality offingers, or the gate bus.
 4. The semiconductor device of claim 3,further comprising a second gate pad coupled to the gate pad on an endof the gate bus opposing an end to which the at least one gate pad iscoupled.
 5. The semiconductor device of claim 3, wherein the source padis coupled to a substrate of a second transistor forming a cascodedevice.
 6. The semiconductor of claim 5, wherein the second transistoris a silicon metal-oxide semiconductor field effect transistor(Si-MOSFET) having a Si substrate.
 7. The semiconductor of claim 6,wherein the Si substrate of the Si-MOSFET is coupled to a conductivefilm and the conductive film is coupled to a drain of the Si-MOSFET. 8.The semiconductor device of claim 5, wherein gate vias are used tocouple the source pad to the second transistor.
 9. The semiconductordevice of claim 5, wherein the gate bus and the gate pad are covered bya passivation layer and the source pads and the drain pads are openthrough the passivation layer.
 10. A semiconductor device comprising: afirst cell comprising a first plurality of fingers and a second cellcomprising a second plurality of fingers; a first drain pad coupled overthe first plurality of fingers; a second drain pad coupled over thesecond plurality of fingers; a source pad coupled over the firstplurality of fingers and the second plurality of fingers; a gate buslocated between the first plurality of fingers and the second pluralityof fingers; and at least one gate pad coupled over the gate bus.